Geothermal circulation system

ABSTRACT

The invention relates to a method for configuring a geothermal circulation system in a target underground region, comprising at least one injection wellbore ( 1 ) and at least one production wellbore ( 2 ), a hydraulic connection being created between the injection wellbore ( 1 ) and the production wellbore ( 2 ) via a crack system ( 4, 4′ ) in the target underground region, and a heat transfer medium being circulated via the injection wellbore ( 1 ), crack system ( 4, 4′ ), and production wellbore ( 2 ), comprising the steps of creating a plurality of cracks ( 4, 4′ ) in the target underground region by the incremental hydraulic separation of short wellbore sections and hydraulic crack generation starting from the respective separated wellbore section. The invention further relates to an arrangement for a geothermal circulation system in a target underground region, comprising at least one injection wellbore and at least one production wellbore, wherein a hydraulic connection is established in the target underground region between the injection wellbore and the production wellbore via a crack system, wherein the injection wellbore and the production wellbore in the target underground region have wellbore segments that are deflected at angles (α, β) of 0° to 80° to the horizontal line, and a plurality of cracks are provided for the hydraulic connection between the injection wellbore and production wellbore.

The invention relates to a method of constructing a geothermal circulation system in low-permeable rock of the deep substratum, the target substratum area, consisting of at least one injection wellbore and at least one production wellbore, wherein a hydraulic connection is produced between the injection wellbore and production wellbore via a hydraulically generated crack or rift system in the target substratum area, and wherein a heat transfer medium is circulated through the injection hole, crack system and production wellbore. Furthermore, the invention relates to an arrangement for a geothermal circulation system in low permeable rock, the target underground area consisting of at least one injection wellbore and at least one production wellbore, wherein a hydraulic connection exists between the injection wellbore and production wellbore via a crack system in the target substratum area.

The low permeable rock of the deep substratum is by far the greatest geothermal resource which could provide, even in countries such as Germany, which are poor in conventional useable geothermal reservoirs, a substantial contribution to heating and electricity. Internationally, research has been ongoing for 35 years working on methods to exploit them. For example, in the so-called “Hot Dry Rock” research projects circulation systems were established at depths of 2000-5000 meters in hot and dry granite. Therein, at depths of several hundred meters, water was injected with very high flow rates and pressures into long uncased wellbore routes to induce a crack system. With this procedure, so far only inadequate flow rates and a correspondingly low thermal capacity of the circulation system are achieved. In addition, unfortunately no sufficiently large heat exchange surfaces could be generated. The generated crack system would essentially be limited to one main crack. The high geothermal potential in Germany thus faces a problem in the creation of an economically feasible circulation system.

Beginning with the previously tested “Hot Dry Rock” concepts, it is therefore an object of the invention provide a geothermal circulation system, which can be economically implemented in engineering-predictable manner.

This object is achieved with a method of forming a geothermal circulation system according to claim 1. Furthermore, with regard to a device, the task is solved by an arrangement according to claim 10.

The formation of the circulation system with a large number of cracks is based on the reasoning, that there exists in the underground a tension anisotropy, whereby hydraulically induced cracks preferably form orthogonal to the smallest principal stress. In general, the vertical mechanical stresses are high and the horizontal stresses are low. Furthermore, in the horizontal stresses there is a pronounced directional anisotropy, whereby hydraulically induced cracks are preferably oriented vertical and, due to the anisotropy of the horizontal stresses, are substantially oriented in the direction of maximum horizontal stress. If now the injection and production wellbore drilling are redirected preferably in the direction of minimum horizontal stress, then cracks can be formed vertical and perpendicular to the reoriented direction of the wellbore propagating from the respective wellbores. These cracks transect with the respective wellbores, depending on the angle of the deviated wellbore segment over a relatively short drill hole section. Similarly, in the case of a wellbore deflected over a longer distance, numerous substantially parallel and vertically oriented cracks can be produced.

In rare cases, the vertical stress in the substrata is the minimum principal stress, for example in areas with pronounced thrust fault tectonics. In this case, the hydraulically induced cracks will preferably orient in a horizontal plane, that is, orthogonal to the minimum principal stresses. Thus, the wellbore lines in the injection wellbore and production wellbore need not to be deflected. Therewith the stepwise crack induction can occur directly from the vertical wellbore track for the production of many parallel and, in this case, essentially horizontal cracks.

For crack generation, preferably a dual packing, guided on a tube arrangement, is used, which allows the hydraulic separation of a short wellbore section, and which can be pressed hydraulically targetedly in the separate wellbore sections. By the relocation of the dual packing, thus, in turn, the separation and the number and desired cracks can be produced. Therewith a multi-crack system in the subterranean area can be produced in an engineering-predictable manner.

Since the injection and/or production wellbore (s) can be piped and cemented prior to the hydraulic crack generation, and sine perforations can be provided in the piping at each segment at which hydraulic crack generation is to be performed, a wellbore section can be prepared with a smooth and uniform wall, which can be reliably hydraulically separated by the dual packing arrangement. By deliberately providing perforations through the casing and cement, the hydraulic connections to the target underground area are created in precisely defined intervals. “Piping” means laying pipe along the whole wellbore while drilling, or only partial sections of the wellbore, which could also be referred to as the “liner”.

If the injection wellbore and production wellbore are drilled and deflected so that they essentially form a horizontal wellbore segment, and run antiparallel to each other at a predetermined depth, then for any of the cracks there exists essentially the same pressure differential between injection wellbore and production wellbore, so that the flow rates at each crack are comparable with each other. For antiparallel wellbores, the pressure losses which are a function of line length are balanced out in the case of substantially similar hydraulic crack characteristics, whereby the flow of the heat transfer medium is evenly distributed to the cracks. Thermal short circuits are thus avoided. Therein “essentially horizontal” means that the wellbore lines have a maximum slope of 20° and are preferably from 10° to horizontal. In a preferred embodiment, the boring of the injection wellbore and the boring of the production wellbore are drilled close to each other in wellbore site, and are so oriented, that the separation of the drilled wellbore runs is made with reference to each other.

Since the cracks, generated hydraulically in the target underground area with a fault zone, propagate from both the injection wellbore and production wellbore, so that the generated cracks extend from each wellbore to the fault zone and thus establish a hydraulic connection between the injection wellbore cracks, the production wellbore cracks and the fault zone, a fault zone located in the underground target area can be integrated into the hydraulic circulation.

If an initial or preliminary drilling of the wellbores, particularly the injection wellbore, is drilled first, and hydraulic cracks are generated from this preliminary wellbore, it would be possible to already conduct geological and geophysical tests for underground exploration in the preliminary wellbore, and then upon determination of suitability, generate the cracks in the target underground area wellbore segment.

If, after the creation of the large number of cracks, these are to be passed through by a second wellbore of the wellbore system, then it is possible to take into consideration, when deciding where to locate of the second wellbore, the precise structure or formation of the hydraulically generated cracks, which had been previously detected by the geophysical measurement techniques. In the boring-through of the cracks with the second wellbore, usually a sufficient hydraulic connection created, so that the geothermal circulation system, comprising injection wellbore, crack system, and production wellbore, is operational. The heat transfer medium, usually water, can now be introduced into the injection wellbore under pressure, and extracted via the production wellbore. Here, the heat transfer medium can circulate at flow rates of 10-200 l/s, in particular 30-100 l/s. At circulation rates of 100 l/s with a rock temperature of 160° C., a geothermal power plant can be operated with an output of approximately 4 MW_(el). In addition, such a geothermal circulation system is useful for heating and/or air conditioning, for example for the heating of living spaces by remote heating systems.

In addition or as an alternative embodiment the second wellbore is lined or encased and cemented, in accordance with which the wellbore casing is perforated at predetermined wellbore sections, and from there a selective hydraulic crack generation takes place from this second wellbore. In this design, therefore, hydraulic cracks are also generated propagating from the second wellbore, which sufficiently link or network with the hydraulic cracks emanating from the initially drilled wellbore, so that the required hydraulic connections for the circulation system are achieved.

Since the wellbores of the injection wellbore and production wellbore in the target underground area run essentially parallel or antiparallel to each other, the flow paths along the cracks in the target underground area to be bridged are very comparable. However, as a result of the parallel running of the holes of the injection wellbore and production wellbore in a multi-crack system, there is the problem that, because of the path-length dependent pressure losses along the length of the wellbores, in the case of same hydraulic characteristics in equivalent cracks, those cracks which have the shortest line length, that is, lie closest to the energy producing unit, conduct through themselves the highest volume flows. The more distant cracks accordingly experience less flows. There is thus a risk that a thermal short circuit will be produced, whereby the total thermal capacity of the generating plant decreases. Particular preference is therefore the formation of antiparallel arranged horizontal wellbore sections, since each of the cracks would have the same pressure difference between injection wellbore and production wellbore. Accordingly, the flow rates of each of the cracks would be comparable. A thermal short circuit is avoided. Alternatively, to compensate for the significant flow resistance in the drill hole of the inclined or horizontal wellbore sections these sections can be designed to gently come closer together in the bed of the wellbore. This ensures that, despite the drop in hydrodynamic pressure gradient across the length of the wellbore, the total flow rate is more evenly distributed among the individual cracks, and besides this, also in the cracks more remote from the wellbore bed have available a greater thermal exchange surface area.

The angle relative to horizontal of the deviated wellbore routes, hereafter referred to as deviating sections, is preferably 0° to 60°, 0° to 45°, in order to achieve a path that, while as short as possible, passes through each individual vertical crack. Accordingly, for the same length of the deviated wellbore track, a larger number of cracks with a given distance from each other can be produced, the greater the deflection of the wellbore from horizontal. In the case of a target area of a geological layer with relatively low power, for example, only a few 100 m, then a low angle to the horizontal of from 0° to a maximum of 15° is preferred.

To limit the resistance to flow in the individual cracks and to achieve a reasonable life with acceptable spacing between the deflected segments of the two wellbores, for a desired total flow rate of 10-200 l/s, preferably 100 l/s, at least 6 cracks, preferably 20 to 50 cracks, are to be provided.

To ensure an overall useful life of at least 20 years for geothermal exploitation of the deposit, and thus to be able to cool down a sufficiently large underground volume, the horizontal or inclined wellbore segments of the injection and production wellbore(s) have a length of 300 m to 2,000 m, preferably 500 m to 1,200 m and the distance between the two wellbores is about 200 m to 1,000 m, especially about 500 m.

In the following, four embodiments of the invention will be presented with reference to the accompanying figures.

Therein there is shown:

FIG. 1 a schematic cross-sectional view of a geothermal circulation system in a first embodiment,

FIG. 2 a schematic cross-sectional view of a geothermal circulation system in a second embodiment,

FIG. 3 the in a cross-section diagram illustrated embodiment of a circulation system shown in FIG. 2, in a view perpendicular to the drawing plane in FIG. 2,

FIG. 4 a detailed view of a segment of a drill hole with dual packing,

FIG. 5 the situation depicted in FIG. 4, during or after production of a crack,

FIG. 6 the situation shown in FIG. 5, with a further crack,

FIG. 7 a further embodiment of the geothermal circulation system,

FIG. 8 a, b a third embodiment of the geothermal circulation system, and

FIG. 9 a, b a horizontal and vertical projection of a fourth embodiment of the circulation system with two anti-parallel disposed running holes.

FIG. 1 shows a first embodiment of a geothermal circulation system. The circulation system comprises an injection wellbore and a production wellbore 2. The injection wellbore extends from the surface 0 to a target underground area Z, which may beat, for example, a mean depth z of in 4000-5000 meters. In the target underground area Z is the injection wellbore 1 is deflected, and runs at an angle β of for example 30° to the horizontal H along a first wellbore segment 11 to the first wellbore bottom 12 of the injection wellbore 1.

At least the first wellbore segment 11 of the injection wellbore has a piping 13, which is provided with cementation 14, as shown in the detailed view in Fig. Along the first wellbore segment 11 the piping 13 has perforations 15 at the desired crack spacings d.

In FIG. 4 a dual packing 3 is shown in the first wellbore section 11, which is comprised of a first packing 31 and a second packing 32 spaced apart therefrom. The dual packing 3 is lowered via pipe tube arrangement 33 from the surface 0 through injection wellbore 1 to the desired position. There, the corresponding wellhole section is hydraulically separated or isolated by the expanding of the two packings 31, 32. Water under high pressure and at a flow rate sufficient to produce a crack is introduced into the pipe tube arrangement 33 and is injected from there through the perforations 15 into the target underground area Z. Accordingly, a substantially vertical crack forms, the surface area of which is oriented substantially orthogonal to the minimum horizontal stress in the target underground area Z. FIG. 5 schematically shows a corresponding crack. After creating the crack, the hydraulic properties of the crack are determined by observing the pressure drop or by observing the return flow rate following relaxation. Following pressure equalization and detensioning of the two packings 31, 32, the dual packings 3 are moved, along with the tubular rods of the pipe tube arrangement 33, so that the perforation 15 in the piping 13 is located and set again at the next adjacent site of crack formation. After hydraulic separation or isolation of the corresponding wellbore section, a further crack 4 is created as is shown in FIG. 6, by again pumping in water under high pressure and with sufficient flow rate through the pipe tube arrangement 33.

After generating a large number of cracks, which for example in the embodiment shown in FIG. 1 are N=13, the production wellbore 2 is drilled. Therein, the production wellbore 2 is also deflected into the target underground area Z, whereby a second drill hole inclined track 21 is formed, extending through the cracks 4 previously located by the geophysical methods. In the area of the intersection of the production wellbore 2 with the cracks 4 there is formed a hydraulic connection, so that the geothermal circulation system is functional. The second bore hole section 21 of the production wellbore 2 terminates after the intersection with the last crack 4 and there forms a second wellbore bottom 22. The second bore hole 21 is preferably oriented parallel to and disposed a distance of, for example, a 500 meters, from the first hole 11 of the injection wellbore. For reinforcing of the wellbore segment 21 this can be provided with a slotted casing or liner.

FIG. 2 shows a second embodiment of the geothermal circulation system in schematic cross section. Functionally identical components bear the same reference numerals as the embodiment of FIG. 1. In contrast to the embodiment of FIG. 1, in FIG. 2 cracks 4, 4′ are generated by both wellbore sections 11 and 21, which cracks intersect in the target substrate area Z. This situation is illustrated in FIG. 3 in a view that is vertical to the plane of FIG. 2, and to the left of the view of FIG. 2. In the embodiment of FIG. 2, the first wellbore segment 11 and the second wellbore segment 21 are again arranged essentially parallel to each other and with separation. The angle of inclination α, β to the horizontal H is likewise about 30°.

A further embodiment of the geothermal circulation system is shown in FIG. 7. Functionally identical components bear the same reference numerals as the embodiment of FIGS. 1 and 2. In the embodiment of FIG. 7, the injection and production wellbores 1, 2 deflected to the extent that the first and second wellbore sections 11, 21 are horizontal within the target underground area Z. This version is preferred in target underground areas Z with relatively low power.

A third embodiment of the geothermal circulation system is shown in FIG. 8, wherein a geological fault zone S is present in the target underground area Z. The presentation is similar to the presentation of the first embodiment in FIG. 3, in a wireframe view. Herein functionally equivalent components are given the same reference numbers as in the above described embodiments. In this third embodiment as shown in FIG. 8, the cracks 4, emanating from the injection wellbore 1, are so constructed that they extend into the fault zone S. The production wellbore 2 is arranged on the other side of the fault zone S. Cracks 4′ are also produced extending from this production wellbore 2, and are likewise so arranged that they extend into the fault zone S. The hydraulic connection between the injection wellbore 1 and the production wellbore 2 is therefore constituted by the connection between the first crack system 4 until the fault zone S and from the fault zone S on the crack system 4′. An advantage of this design is that the crack propagation in each case need only extend up to the fault zone S located between the wellbores. It is not necessary that the two crack systems 4 and 4′ extending from the wellbores intersect directly, rather, the connection between the various crack systems 4 and 4′ is achieved via the fault zone S. The fault zone S thus acts also as a flow director and therefore as a heat exchange surface, so that the total effective heat exchange surface area of the geothermal circulation system is expanded by the already existing underground geological fault zone S. The efficiency of the circulation system of the fault zone can thus be improved by taking advantage of the fault system S.

A fourth embodiment is shown in FIGS. 9 a and 9 b. It can be seen, from the vertical view 9 a of two holes, that the injection wellbore 1 shown in solid line begins at the Earth's surface at a bore location with the coordinates of 0,0. The wellbore, at a depth of 1000 m, has a deflection to the south, so that the wellbore continues at an incline to a depth of about 3500. There it is deflected in the opposite direction, that is, to the north, and at a depth of 4400 m transitions to a horizontal wellbore segment 11 with a length of about 1000 m.

The production wellbore 2 shown in dashed line in FIG. 9 a is produced in mirror image. As can be seen from the horizontal view in FIG. 9 b, as a result of the deflections in the southern or, as the case may be, northern direction of the two wellbores, as well as a lateral deflection in east and west directions, a spacing of 400 m results between the two antiparallel oriented wellbore sections 11, 21.

This antiparallel arrangement of the horizontal wellbore sections 11, 21 ensures that the flow path from the surface through the injection wellbore 1, its horizontal wellbore section, cracks (not shown) bridging across the horizontal distance between the wellbore sections 11, 21, as well as the corresponding section in the production wellbore 2, always has the same length—regardless of the cracks being flowed through—so with approximately the same flow resistance is expected for each flow path. According, the heat transfer medium distribute itself substantially uniformly in cracks extending between the horizontal wellbores 11, 21, so that the whole rock volume is thermally evenly utilized.

Alternatively, for balancing out the flow resistance, an arrangement can be selected, in which the two wellbore sections 11, 21 are arranged running essentially the same direction to each other, wherein both boreholes gradually narrow going towards their wellbore bottoms 12. Therein, the higher flow resistance of the heat transfer medium due to the longer flowpath through crack connecting the two wellbores can be offset against the reduced flow resistance between the two wellbores in the vicinity of the wellbore bottoms. At the same time, by these measures the area of the cracks being flowed-through in the region distant from the wellbore bottoms is increased, and therewith a more rapid cooling of these cracks is counteracted.

LIST OF REFERENCE NUMERALS

-   1 Injection wellbore -   11 First wellbore segment -   12 First wellbore bottom -   13 Piping or casing -   14 Cementing -   15 Perforation -   2 Production wellbore -   21 Second wellbore segment -   22 Second wellbore bottom -   3 Dual packing -   31 First packing -   32 Second packing -   33 Pipe tube arrangement -   4,4′ Crack -   α, β Angle -   a Distance -   d Crack distance -   H Horizontal -   N Number of cracks -   O Surface -   z Depth -   S Fault zone -   Z Target underground area 

1. A method of forming a geothermal circulation system in a target underground area (Z), the method comprising: forming at least one injection wellbore (1) and at least one production wellbore (2), forming a hydraulic connection between the injection wellbore (1) and the production wellbore (2) through a crack system comprising a number of cracks (4, 4′) in the target underground area (Z), and circulating a heat transfer medium through the injection wellbore (1), crack system (4, 4′) and production wellbore (2), wherein said number of cracks (4, 4′) in the target substrate area (Z) is created by stepwise hydraulic separating or insulating short hydraulic wellbore segments and generating cracks hydraulically, said cracks emanating from the respectively separated or insulated wellbore segments.
 2. The method according to claim 1, wherein said injection and production wellbores (1, 2) are so deflected from vertical during the drilling, that the wellbores pass through the target substrate area (Z) at an angle offset from vertical up to horizontal.
 3. The method according to claim 1, wherein the injection and/or production wellbore (s) (1, 2), prior to hydraulic crack generation, are lined and cemented, and that a perforation (15) is provided in the piping or lining (13) associated with each hydraulic crack generation.
 4. The method according to claim 1, wherein the injection wellbore (1) and the production wellbore (2) are drilled and deflected in such a manner that they include essentially horizontal wellbore sections (11, 21), which run antiparallel to each other at a predetermined depth.
 5. The method according to claim 1, wherein, in a target underground area (Z) with a fault zone(s), hydraulic crack generation takes place from the injection wellbore (1) as well as the production wellbore (2), so that the generated cracks extend from the respective wellbore into the fault zone (S) and therewith establish a hydraulic connection between the injection wellbore (1) and the production wellbore (2) via the fault zone (S).
 6. The method according to claim 1, wherein a first borehole, particularly the injection wellbore (1), is drilled first, and hydraulic crack generation takes place from this initial drill hole, and after the creation of a number of cracks (4), these are passed through by a second borehole, especially the production wellbore (2).
 7. The method according to claim 1, wherein a first borehole, particularly the injection wellbore (1), is drilled first, and wherein a second borehole, especially the production wellbore (2) is sunk in such a manner, that by generating a large number of cracks (4, 4′) emanating from the two boreholes, an intersecting crack system, and thus hydraulic connection between the two boreholes, is produced.
 8. The method according to claim 6, wherein the second borehole is lined or encased and cemented, after which the piping (13) is perforated at dedicated drill hole sections and there a selective hydraulic crack formation takes place out of this second borehole.
 9. The method according to claim 2, wherein the crack generation is performed using a dual packing guided on a tube pole within the perforated piping or casing (13).
 10. A well for extracting geothermal energy via a geothermal circulation system in a target underground area (Z) consisting of at least one injection wellbore (1) and at least one production wellbore (2), with a hydraulic connection between the injection wellbore (1) and the production wellbore (2) provided by a crack system (4.4′) in the target underground area (Z), wherein the injection wellbore (1) and the production wellbore (2) in the target underground area (Z) include wellbore segments (11, 21) deflected at an angle (α, β) of from 0° to 80° to the horizontal (H), and a number of cracks (4, 4′) are provided for the hydraulic connection between injection wellbore (1) and production wellbore (2).
 11. The well according to claim 10, wherein the wellbore segments (11, 21) of the injection wellbore (1) and the production wellbore (2) in the target underground area (Z) run substantially parallel to each other or at an acute angle to each other, so that the distance (a) between the holes towards the bore bottom (13, 23) decreases, or are arranged antiparallel.
 12. The well according to claim 10, wherein the angle (α, β) is from 0° to 60°, in particular from 0° to 45°, preferably 0° to 20°.
 13. The well according to claim 10, wherein at least six cracks (4, 4′), preferably 20 to 50 cracks, are provided.
 14. The well according to claim 10, wherein the injection wellbore (1) a casing (13) and cementation (14) are provided, wherein perforations (15) are provided in the casing (13) at the intended sites of crack formation.
 15. The well according to claim 10, wherein the production wellbore (2) has a cementing and casing, and wherein perforations are provided in the casing at desired sites of crack formation. 